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Needing Help With Cobra 29 Keeps Burning Up D8

Discussion in 'CB Radio Modifications' started by blake25537, Oct 31, 2020.

  1. blake25537

    blake25537 Active Member

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    I have a Cobra 29 and when I put power to it, it'll burn up D-8. I've checked a few things around it but I have no idea where to look now. I'd very grateful for any help finding the problem.

    Thanks Again.


     

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  2. Dmans

    Dmans Sr. Member

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    Maybe this??
    Inked29d23_LI.jpg
    73
    David
     
    Shadetree Mechanic likes this.
  3. Eldorado828

    Eldorado828 8-2-8 in the Lonestar State

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    Drop a slice of bread in that bad boy, hit the switch and you've got toast.
     
    sp5it likes this.
  4. blake25537

    blake25537 Active Member

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    I replaced the one that was clipped and D8 but it still burns it up when power is on

     
  5. Dmans

    Dmans Sr. Member

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    blake25537,
    If you have a schematic for your particular version of 29 (I.e. 29LTD, 29 LX LE, 29NW ST etc) you could check all items tied in with D8. The audio transformer (T1) and a shorted PA jack are the first things that come to my mind.
    Any modifications to the unit? Talkback added or anything else audio related?

    73
    David
     
    blake25537 likes this.
  6. nomadradio

    nomadradio Analog Retentive

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    That's because the main power is feeding into one end of D8 where it comes out of the modulation transformer. D8 feeds out into the final and driver transistors. Something in that section has a short to ground.

    A multimeter will reveal a short to ground, using the lowest ohms range. Black lead to the negative pin of the power socket, red probe to the banded end of D8, or to the foil trace where it connects.

    Unsolder and separate the center leg of the final transistor and see if the short goes away.

    If not, unsolder and separate the center leg of the driver transistor and check.

    If you still have a short-circuit reading to ground, one of the parts still connected to the foil trace at the banded end of D8 is what's shorted. Follow the foil trace to each remaining part. Unsolder and isolate each one by one. Eventually the short reading will clear when the failed part is disconnected.

    73
     
  7. blake25537

    blake25537 Active Member

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    Dmans,

    LTD Classic no mods or talkback looks stock to me. Sorry no schematic to the radio also.

    Thanks

     
  8. blake25537

    blake25537 Active Member

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    nomadradio,

    Thanks I'll start check and see what I can find.

     
  9. Dmans

    Dmans Sr. Member

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    #9 Dmans, Oct 31, 2020
    Last edited: Oct 31, 2020
    blake25537 and 144inBama like this.
  10. blake25537

    blake25537 Active Member

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    Thanks to You both Dmans and nomadradio the Cobra 29 lives again. With the info you all gave me I tracked the problem down. The final was shorted out, I tested as explained and sure enough thats what it was. I installed a new final and she's like new again. Many thanks to you both......
     
  11. blasphemy000

    blasphemy000 Active Member

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    What's the date of manufacture sticker on the back of that's 29 say? It looks like they tidied up the PCB layout and respun it. Looks much neater inside than the normal design I'm used to looking at.
     
  12. blake25537

    blake25537 Active Member

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    Feb 2010 yes it looks a lot better this way lol
     
  13. blasphemy000

    blasphemy000 Active Member

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    Weird. I was figuring that was a newer board. I have a few from 2013 through 2017 and they all have the standard old style looking board. Very strange.
     
  14. nomadradio

    nomadradio Analog Retentive

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    Congratulations for sticking with it and getting the radio back on the air!

    Gotta love it when a plan comes together.

    73
     
    Tokin and Shadetree Mechanic like this.
  15. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    That's some good stuff right there. Glad you got it going.
     

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